It is very common to build electronic systems which are designed to have one or more component boards plugged into them. In many such systems, such as in many computer systems, there is a common bus, comprised of a plurality of parallel conductors. A plurality of usually parallel bus connectors are mounted on the bus and the electronic component boards can be plugged into such a bus. Usually the parallel conductors of the bus are contained on a printed circuit board, which is called the backplane of the system. Usually the parallel bus connectors are mounted in a parallel manner perpendicular to the parallel conductors of the bus. And usually each component board is also formed of a printed circuit board which has connectors on one edge, which connectors are designed to mate with the bus connectors. Often the backplane printed circuit board is mounted in some sort of box or frame, which is designed to give support to the backplane. Such frames usually contain card guides designed to hold and give support to one or both side edges of a component printed circuit board when the bottom edge of that board is mated with the bus connectors.
As the software and applications run on computer and other electronic systems become larger, it becomes increasingly desirable to be able to pack a greater number of electronic components into the same volume. This means that it is desirable to be able to place an increased number of components on printed circuit boards. For example, it has been common to increase the number of memory chips that can be placed on memory boards by making boards which have memory chips mounted on both sides of their printed circuit card. It has also been common to make memory boards which are actually comprised of two boards, a mother board which fits into the card guides and bus connectors and which carries chips on one or two sides, and a daughter board which is mounted parallel to the mother board on spacers which stick up from one side of the mother board. Such a daughter board will also have chips on one or both sides.
One of the problems, however, with schemes such as mounting chips on both sides of a board or of using boards with daughter boards is that the space allowed for boards in a given computer or electronic system is often limited. In fact, it is most often limited by the need for a given board to able to fit into a given connector on a bus without encroaching into the space reserved for possible boards in adjacent bus connectors. To make it easier for different companies that manufacture boards used in a given system to fit into adjacent bus slots without touching or blocking each other, manufacturers of computers often announce specifications which limit the size of boards designed to fit into the bus slots of their computers. These specifications state a maximum permissible extent to which components may stick out on each side of the parallel bus connectors on the computer system's bus.
It should be noted that traditionally computer boards have had a front on which all the larger components, such as chips, capacitors, and resistors, were mounted and a back on which no such larger components were mounted. For this reasons the specifications for the size limitations for most boards have been asymmetrical, that is, they specify that the maximum permissible extent to which components may stick out on one side, which we shall call the front, of a bus connector is much greater than the maximum permissible extent to which they may stick out from the other side, which we shall call the back. This is beneficial when all the large components of a board are placed on one side, as was traditionally the case. But it can be a problem if one wishes to place components, such as memory chips, on both sides of a board, since often the thickness of such memory chips exceeds the maximum permissible extent which components are allowed to stick out from the back side of a board according to such bus specifications.
Furthermore, the maximum permissible extent on the front side of boards is often not large enough to allow a board to have one layer of chips mounted on its front side and a daughter board mounted parallel to that front side which also has a layer of chips. This is because chips, which generate heat, require ventilation and, thus, if a daughter board is to be placed over a layer of chips on its mother board it is necessary for there to be a sufficient distance between the chips on the mother board and the daughter board to allow for proper ventilation.